1. Field of the Invention
The present invention relates to a moving coil type encoder motor adapted for example, for speed control (course control) and position control (fine control) in accordance with position signals transmitted from the encoder incorporated with a head positioning actuator of a magnetic disk drive, and particularly, to the constitution of a motor portion of the encoder motor. The "encoder motor" is an apparatus in which an encoder and a motor are integrally formed.
2. Description of the Related Art
Several attempts have been made to use an encoder motor for, for example, a magnetic head positioning system (an actuator positioning system) of a compact magnetic disk drive. Such an encoder motor must be very compact, be capable of a quick acceleration (high-speed access), and have a high rigidity. However, an encoder motor of this type, particularly a moving coil type, which can satisfy the above requirements, has not been developed. Conventional encoder motors are mostly the moving magnet type, which does not substantially satisfy the above requirements, since the moving magnet type encoder motor has the following drawbacks:
(1) A part of a rotary member of the encoder motor is constituted by a yoke and magnets having large mass, so that the total inertia of the rotary member is very large and thus hinders any attempt to improve the speed of the encoder motor.
(2) The eddy current loss and hysteresis loss occurring in a core (a stator) of the encoder motor generate a rotational resistance that also hinders any improvement in the speed thereof.
(3) To improve the speed, the number of coil windings must be increased to increase the torque (electromagnetic force) to be generated, and thus the size of the coil is unavoidably enlarged to make a compact encoder motor not feasible.
(4) Since the rotating mass of the rotary member is large, and since the torque is transferred via the magnets and yoke and a shaft to a code wheel in the encoder motor, the rigidity of the rotary member is low, and the resonance point (natural frequency) thereof is also low. Accordingly, the rotary member is resonated with a low frequency (a low number of oscillations) to cause torsional vibration in the shaft, thereby reducing the speed and controllability of the encoder motor. Namely, with respect to the magnets, i.e., a driving source of the rotational movement, rotational deviations and vibration tend to occur in the code wheel, i.e., an output source of the control signals, and in a capstan, i.e., a final output portion for a controlled member (driven member such as a positioning member). Further, bearings for supporting the shaft are held by a cover and a case, which are not integrally formed, so that it is difficult to realize a mechanical accuracy of the portions holding the bearings. Accordingly, the deflection accuracy of the shaft and the squareness of a plane of the code wheel with respect to an axis of rotation of the shaft is not maintained, and therefore, a precise control is not feasible.